1. Field of the Invention
This invention relates to a semiconductor laser device. More particularly, it relates to a GRINSCH (Graded Index-SCH) semiconductor laser device with a superlatticed structure having an optical guiding layer between the active layer and each of the first and second cladding layers.
2. Description of the Prior Art:
In recent years, with the advance of crystal growth techniques, it has become possible to form extremely thin crystals, and quantum well semiconductor laser devices with a superlatticed structure have been developed. The quantum well semiconductor laser devices are advantageous over conventional doublehetero (DH) junction-type semiconductor laser devices in that the said quantum well semiconductor laser devices can attain laser oscillation at a low threshold current level. On the contrary, the quantum well semiconductor laser devices are disadvantageous in that they must have an active layer with a thickness of as thin as 100 .ANG. or less. Such an thin layer has not yet been formed by liquid phase epitaxy, but it has been formed by molecular beam epitaxy or metal-organic chemical vapor deposition. However, when the growth rate of layers in a molecular beam epitaxial growth process or a metal-organic chemical vapor deposition growth process is set at an exceedingly high level, good quality crystal layers cannot be obtained. Accordingly, for example, a growth rate of about 1 .mu.m/hour is typically used for molecular beam epitaxy. In conventional DH-type semiconductor laser devices that are made of the Al.sub.x GA.sub.1-x As system (x is the AlAs mole fraction), the substrate and/or the cap layer generally function as a light-absorbent region by which laser light that has been oscillated from the said semiconductor laser devices is absorbed, and in order to prevent this phenomenon, the thickness of the cladding layers must be set to be as great as possible so that light that has penetrated from the active layer into the cladding layers cannot arrive at the said light-absorbent region such as the substrate and/or the cap layer. For this reason, the thicknesses of both the p-sided and n-sided cladding layers of the conventional DH-type semiconductor laser devices are set at as great as 1 .mu.m or more (e.g., about 1.5 .mu.m). The DH-type semiconductor laser devices are generally provided with the active layer with a thickness of 0.01 .mu.m or less, the cap layer with a thickness of 0.51-1 .mu.m, and the cladding layers with the above-mentioned thicknesses. Moreover, when molecular beam epitaxy is used, the DH-type semiconductor laser devices are occasionally designed to have a GaAs buffer layer with a thickness of 0.5 .mu.m or more. Accordingly, when the DH-type semiconductor laser devices are produced by molecular beam epitaxy, if the AlAs mole fractions of the cladding layers and the active layer, respectively, are set of 0.45 and 0.15 and if the growth rates of the GaAs crystal layers and the Al.sub.0.45 Ga.sub.0.55 As crystal layers, respectively, are set at 0.7 .mu.m/hour and 1.56 .mu.m/hours, it will take at shortest about 3.5 hours to complete the growth of crystals. This means that the productivity of DH-type semiconductor laser devices is slow. Moreover, when molecular beam epitaxy is used, an increase in the thickness of grown layers gives rise to an increase in the density of the surface defects, which causes deterioration of the device characteristics and a decrease in the production yield.
Moreover, in conventional DH-type semiconductor laser devices, the thickness of the active layer is set at as small as about 0.1 .mu.m (preferably 0.07 .mu.m or less) so as to attain laser oscillation at a low threshold current level and attain excellent optical characteristics. In conventional semiconductor laser devices with the DH structure, not only the active layer is made very thin, but also the changes of the refractive index only arise at the interface between the active layer and each cladding layer, so that light does not decay within the active layer but it decays within the cladding layers to a large extent. Thus, in order to prevent detrioration of the device characteristics, as described above, the thickness of each cladding layer must be set at as great as about 1.5 .mu.m. Even when an optical guiding layer is disposed between the active layer and each cladding layer, if the distribution of the refractive index of the said optical guiding layers is uniform within the said optical guiding layers, the changes in the refractive index only arise at the interface between the active layer and each optical guiding layer and at the interface between each optical guiding layer and each cladding layer. Thus, although leakage of light from the active layer into the cladding layer can be suppressed to a certain extent, it still occurs and light exists to a large extent within the cladding layers. Therefore, to make the cladding layers thin causes deterioration of the device characteristics. Especially, in quantum well laser devices that have a very thin active layer by which laser oscillation can be attained at a low threshold current level, the distribution of light is unaffected by the quantum well, so that the distribution of light of the quantum well laser devices becomes nearly equivalent to that of DH-type laser devices with the active layer having a thickness that corresponds to the thickness of [one optical guiding layer] plus [the active layer] plus [the other optical guiding layer] of the said quantum well laser devices. Therefore, a large amount of light leaks from the active layer to the cladding layers, as well, in the quantum well laser devices.